As a technique of detecting a specific substance contained in a sample liquid, a method of detecting fluorescent light from the specific substance is known. With this technique, if the amount of the substance contained is minute in orders of ppm to ppb, the quantity of light is so weak in some cases that the detection thereof may be difficult. Moreover, in the detection of a sample liquid, the fluid volume (liquid height) in a container needs to be measured in some cases. However, it is conventionally difficult to measure a fluid volume with precision especially, for example, when dropping a minute amount (1 mL or less) of liquid into a test container, due to dispersion in dropping fluid volume, evaporation of the solvent, or the like.